1. Field of the Invention
The present invention generally relates to apparatus for stressing a shank part of member or such as a bolt, shaft, or stud, and, in particular, to method and apparatus for facilitating loosening of multiple jack bolt tensioning devices that may be seized due to prolonged exposure to high temperature and/or corrosive atmosphere.
2. Description of the Prior Art
It is a common practice for workmen to stress a bolt by applying torque to the bolt head to advance a threaded portion along threads in a tapped hole or a nut member so that the shank portion of the bolt is placed under a desired or predetermined mechanical stress. The stress imparted to the shank portion of the bolt can be determined by using a torque wrench to measure the torque applied to the bolt or by using other means to measure the applied force by a spanner wrench. The torque may be applied to the head portion of the bolt or to the nut. A stud can be stressed in a similar way by torque applied to a nut engaged with a threaded portion of the stud. The present invention is designed to permit stressing of a large fastener, e.g., a bolt or a stud, having a diameter generally one inch or larger. The magnitude of stress that can be applied to a bolt, stud or similar fastener has limits of practicality, particularly with respect to workmen. For example, a one-inch diameter threaded bolt made of high-strength material can be tightened by the use of a spanner wrench to the elastic limit of the material With coarse threads, a stress to the elastic limit of the material requires about 1000 ft-lb of torque, e.g., 100 pounds of force applied by a 10 foot lever. Under ordinary conditions, this exceeds both the practical length of the lever and the amount of force a workman can deliver to the lever. Similarly, under usual conditions, a workman using a spanner wrench cannot stress the shank of a nut and bolt assembly that is four inches in diameter and having coarse threads to 57,000 ft-lb which is the required torque to stress the bolt to the elastic limit of a typical material comprising the nut and bolt. To develop torque of this magnitude, 5700 pounds of force must be applied to a 10-foot long lever which cannot be accomplished without employing massive machinery or special facilities.
A number of varied methods and apparatus for imparting elevated tensile stresses to shaft members are known. These include, inter alia, thermal shrinkage, hydraulically-powered devices and mechanically operated devices.
Thermal shrinkage is an old method for stressing a bolt. The bolt is first heated to elongate its shank. The bolt is then torqued to a precalculated stress and finally is allowed to cool so that upon cooling, thermal shrinkage tensions the bolt. Usually, this method cannot be used where the amount of stress must be accurately controlled or precisely established. It is also difficult to unscrew the bolt from the threaded member without reheating the bolt.
Hydraulically-operated shaft tensioning devices, due to inherent physical constraints, are limited in their application to usage only in low-to-moderate temperature environments and in instances wherein only moderate tensioning of the shaft is required. This is because the hydraulic seals in such devices are incapable of maintaining their structural integrity beyond comparatively restricted levels of temperature and/or pressure Hydraulically-powered devices are further undesirable because the magnitude of force which they can develop is also restricted by the available mounting space for the device. Even if space is available for mounting a hydraulic tensioning device, in order to impart even moderate tensile stresses to a shank of a large-diameter bolt, or the like, it is typically necessary to develop and maintain hydraulic pressure of about 15,000 psi, which is near the maximum hydraulic pressure attainable by even large-scale industrial equipment and extremely taxing on the hydraulic seals and pressurizing equipment. Yet such pressures are still entirely inadequate for tensioning of a large-diameter metallic shank to a level of stress approaching its elastic limit. Examples of such hydraulic tensioning devices can be found in U.S. Pat. Nos. 3,835,523; 3,841,193; 3,886,707; 4,075,923 and 4,182,215.
Mechanically-operated apparatus such as multiple jack bolt apparatus, particularly, that disclosed in my prior U.S. Reissue Pat. No. 33490, the disclosure of which is incorporated herein by reference, are effective means for applying extremely high tensile stresses to shanks of fasteners. As will become apparent from the instant invention disclosure, it is to such multiple jack bolt tensioning apparatus that the present invention represents a significant improvement.
Multiple jack bolt tensioning apparatus such as that of my prior U.S. Reissue Pat. No. 33490 have been used successfully in most applications where large clamping or tension forces are required There is no difficulty in assembling the bolting systems, or in creating clamping forces of millions of pounds by using very hard jack bolts with the proper lubricants. Moreover, under normal room temperature conditions, there is also no problem in disassembling multiple jack bolt bolting systems by simply unscrewing the jack bolts in established patterns.
However, problems in the disassembly of multiple jack bolt bolting systems can develop if the lubricant used during assembly of the system has broken down or disappeared. The breakdown and disappearance of the lubricant occurs mostly by prolonged exposure of the bolting system to oxidation or corrosion or by exposure to extreme heat. Multiple jack bolt tensioners of the type under discussion have been used for temperatures above 1300 degrees F. (690 degrees C). Petroleum lubricants typically break down above 150 degrees C. and virtually all other lubricants including molybdenum disulphate, graphite and nickel powder lubricants lose their lubricity below 750 degrees F. (400 degrees C.). For example if 1/2" (12.7 mm) jack bolts are tightened at 100 ft. lb. (135 N-m), it takes only 90 ft-lb (122 N-m) to loosen them as long as the original lubricant is intact. If, however, the bolting system is heated to 1000 degrees F. (538 degrees C.) it takes 120 to 200 ft-lb (163 to 271 N-m) to break the jack bolts loose depending on the lubricant that was used. Near the high end of the "break loose" torque, the jack bolts may twist off before they break loose. The same thing can happen when a multiple jack bolt tensioner has been exposed to corrosive or other forces that have destroyed the lubricant used during the assembly operation.
The aforementioned U.S. Pat. No. 4,182,215 discloses a hydraulic system for releasing a bolt tensioner apparatus. However, for reasons noted hereinabove, such apparatus is entirely useless in extremely high temperature environments.
A further disadvantage caused by usage of multiple jack bolt tensioning apparatus in high temperature and corrosive environments is that the tremendous point loads exerted by the thrust ends or tips of the jack bolts often cause permanent deformation of the structural flange, thrust washer, and the like, against which they are compressively abutted.
An advantage exists, therefore, for simple and inexpensive method and apparatus for ensuring disassembly of multiple jack bolt bolting systems, particularly systems whose jack bolt lubricant has broken down or been lost due to exposure to extreme heat and/or prolonged oxidation or corrosion.
A further advantage exists for a means to prevent permanent deformation of structural flanges, thrust washers, and the like, caused by extreme point loads exerted by the thrust tips of the jack bolts of a multiple jack bolt tensioning systems, particularly in systems exposed to high temperature and/or corrosive atmosphere.
It is, therefore, an object of the present invention to provide a simple and inexpensive method and apparatus for ensuring disassembly of multiple jack bolt bolting system, particularly systems whose jack bolt lubricant has broken down or disappeared due to exposure to extreme heat and/or prolonged oxidation or corrosion.
It is a further object of the present invention to provide means to prevent permanent deformation of structural flanges, thrust washers, and the like, caused by extreme point loads exerted by the thrust tips of the jack bolts of a multiple jack bolt tensioning systems, particularly in systems exposed to high temperature and/or corrosive atmosphere.
Still other objects and advantages will become apparent in light of the attached drawings and written description of the invention presented hereinbelow.